Hydraulic pressure can be used for transmitting large power or force via a free path and for transmitting signals in accordance with the magnitude of the pressure and is thus used in various apparatuses. For example, hydraulic pressure is used for switching or setting transmission paths of power or securing a transmission torque capacity in a power train of a vehicle. Japanese Patent Application Publication No. 2010-151240 (JP 2010-151240 A) discloses an example of such a kind of hydraulic pressure control device. The device disclosed in this publication is a hydraulic pressure control device for a belt type continuously variable transmission which is configured such that a hydraulic pressure chamber (hydraulic actuator) is provided in each of one pulley (primary pulley) and the other pulley (secondary pulley) on which a belt is wound, hydraulic oil is supplied to or discharged from the primary pulley to change the groove width thereof and to thereby set a gear ratio, hydraulic oil corresponding to a demand drive amount represented by an accelerator operation amount or the like is supplied to the secondary pulley to set belt squeezing force corresponding to the torque to be transmitted.
Further, a supply-side electromagnetic switch valve and a discharge-side electromagnetic switch valve are connected to the hydraulic pressure chamber of each of the pulleys. The hydraulic circuit including the switch valves is configured with through holes, groove sections, and so forth that are formed in a rotational shaft, a fixed section supporting that, and so forth. Thus, the hydraulic circuit includes a section formed over two members that relatively rotate or slide in an axial direction. A prescribed clearance is formed between the two members for their smooth relative movement (relative motion). This may result in leakage of hydraulic oil from the clearance between the two members in a portion where the hydraulic circuit crosses the two members. Accordingly, a seal member is provided to the clearance in order to secure liquid tightness or air tightness, similarly to one in a common hydraulic apparatus.
The seal member is to tightly fit on each of the two members that relatively move and to thereby restrict leakage of hydraulic fluid. The seal member is thus formed of a material having high wear resistance and as small frictional resistance as possible so that the seal member is suitable for its original function. This kind of seal member is made tightly fitted on the two members not only by own elastic force but also pressure from a fluid (or hydraulic oil) to be sealed. Therefore, despite the small friction coefficient, high pressure for tight fitting or a high relative speed between the two members may result in a temperature increase due to frictional heat. This may cause a decrease in durability of the seal member or a decrease in durability of the fluid contacting the seal member.